Good floor planning would include like avoid running digital lines under the devices, analog ground plane can be allowed. Fast switching digital signal should be shielded with digtial ground to avoid radiating noise to other section of the board and it should never run near analog signal paths. rossover of digital and analog signal should be avoided.

You can have 2 separate ground planes and make provision for multiple connection on each plane then connect the the two planes on one center point or you may connect to a multiple points but please ensuure that the multiple connection should mimic a solid GND plane.

The MT-031 app note is a nice article to start with. The article discusses the general grounding philosophy for converters. It presented different grounding options and depends on teh application and characteristic of the mixed signal device.

Now for the AD7656, a solid GND plane is acceptable as long as a good floor planning is used to distance fast switching digital devices from high perfromance analog.

Can you please elaborate on the meaning of the term "floor planning" .

I thought about using the AD7656 datasheet recommendation of two separate planes, ADND and DGND, where DGND is the card massive GND copper layer and AGND is a big copper "Island" coming from the grounding pins of a separate isolated DC-DC unit which generates my 5V_A and 12V_A (another isolated DC-DC unit, with separated Grounding and power is responsible for my 5V_D and DGND).

However, I have quite few ADC and DAC on my board. What should I do with the recommended short between AGND and DGND?

Put one short resistor below one of the ADC?

Put several short resistors, one below each of the ADC?

Put one short resistor in the middle of the Analog area to serve all ADC/ DAC?

Good floor planning would include like avoid running digital lines under the devices, analog ground plane can be allowed. Fast switching digital signal should be shielded with digtial ground to avoid radiating noise to other section of the board and it should never run near analog signal paths. rossover of digital and analog signal should be avoided.

You can have 2 separate ground planes and make provision for multiple connection on each plane then connect the the two planes on one center point or you may connect to a multiple points but please ensuure that the multiple connection should mimic a solid GND plane.